1. Field of the Invention
The invention generally relates to multimedia applications and, more particularly, to the encoding and streaming of video information over a communication network.
2. Discussion of Related Art
Generally speaking, there are two modern approaches to "playing-back" multimedia information located at a remote location, such as playing-back a "video clip" on the Internet. The first approach is to have a client node download a file having the video information from a corresponding "website," or server node, and to then play-back the information, once the file has been completely transferred. The second approach is to have the sever node "stream" the information to the client node so that the client may begin play-back soon after the information starts to arrive. Because the streaming approach does not suffer from the long start-up delays inherent in the downloading approach, it is believed to be preferable in certain regards.
It is believed that a substantial number of remote access users, such as Internet users, access the network via a voiceband modem. To this end, various communication standards have been proposed. H.261 and H.263, for example, each specify a coded representation that can be used for compressing video at low bitrates. (See ITU-T Recommendation H.263 of 2 May 1996, which is hereby incorporated by reference in its entirety.)
Because typical voiceband modems have maximum data rates of less than 56 Kb/s, the quality of a streamed play-back depends on how effectively the channel is used. TrueStream.TM. Streaming Software, version 1.1, for example, keeps the channel at full utilization to improve the play-back's appearance. (TrueStream.TM. Streaming Software, version 1.1, is available from Motorola, Inc.)
In short, with version 1.1 of the TrueStream.TM. Streaming Software, a target data rate is first selected, for example, 20 Kb/sec for a 28.8 Kb/sec modem, the other 8.8 Kb/sec of bandwidth being saved for audio information and packet overhead. If a sequence of video frames is to be encoded and because of its inherent informational content the streaming of the encoded data would require a data rate higher than the target rate, then the TrueStream.TM. system adjusts certain encoding parameters so that encoded frames require less bits. On the other hand, if a sequence of video frames is to be encoded such that the streaming of it would not fully utilize the channel, the TrueStream.TM. system applies a "use it or lose it" approach and adjusts certain encoding parameters to improve the video quality and so that the channel capacity is used. The consequence of the above is that in the former case the sequence will be played-back with pictures having a relatively coarser level of detail and a relatively smaller frame rate, and that in the latter case, the sequence will be played-back with pictures having a finer level of detail and a higher frame rate.